INTERNAL ANATOMY OF INSECTS. 93 



by retrograde movements of the currents, or by oscilla- 

 tory motions of the blood-globules. 



In proportion as the wings are developed, the circula- 

 tion in the laminae diminishes, and ultimately ceases, pre- 

 paratory to the detachment of the laminae themselves. 

 At the same time, however, it presents itself under a new 

 form in the wings. In these the excurrent or arterial 

 stream takes its course along the inner margin of the 

 wing, and the recurrent or venous returning along the 

 outer; whilst, occasionally, other transverse currents take 

 their course through the net- work of the wing from its 

 inner to its outer margin. As the wings are further 

 developed, the circulation in them, like that in the cau- 

 dal laminae, gradually becomes weaker and ultimately 

 ceases a . 



The next observations were made on the transparent 

 larva of a neuropterous insect (probably a Semblis or 

 Sialis), in which the pulsations of the dorsal vessel were 

 distinctly seen at its posterior extremity, from which they 

 were propagated towards the anterior ; these two divi- 

 sions of that vessel appearing to bear to each other the re- 

 lation of a heart and aorta. There were no traces of other 

 vessels, though regular and rapid currents of blood-glo- 

 bules, exterior to the tracheae, proceeded from the head 

 towards the posterior extremity of the body, where each 

 of these currents entered the heart, which again propelled 

 its contents with accelerated velocity through the anterior 

 part of the dorsal vessel towards the head. The lateral 

 currents also were accelerated upon each contraction of 

 the heart, proving that they must communicate with the 

 dorsal vessel at the anterior part of the body, though the 



* Introd. to Comp. Anat. ii. 393-. Engl. Trans. 



